powerpc/perf: Export Power8 generic and cache events to sysfs

Power8 supports a large number of events in each susbystem so when a
user runs:

	perf stat -e branch-instructions sleep 1
	perf stat -e L1-dcache-loads sleep 1

it is not clear as to which PMU events were monitored.

Export the generic hardware and cache perf events for Power8 to sysfs,
so users can precisely determine the PMU event monitored by the generic
event.

Eg:
	cat /sys/bus/event_source/devices/cpu/events/branch-instructions
	event=0x10068

	$ cat /sys/bus/event_source/devices/cpu/events/L1-dcache-loads
	event=0x100ee

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>